Switch mode transformers (switching transformers) are used mainly in switching power supplies and DC-DC converters. They provide a storage element for transferring energy from input to output in discrete packets as required in switching power supplies, regulators and converters. Switch mode transformers have several basic topologies: push-pull, flyback, boost, forward, buck and bridge. Push-pull transformers are used in push-pull configuration circuits such as power supplies. Flyback transformers use the flayback or kickback of an inductor to convert an input voltage to a desired output voltage. The input voltage or charging cycle produces energy which is stored in the inductor. The input energy or discharge cycle is then transferred to the output. Boost or step-up switch mode transformers convert a lower DC input voltage to a higher DC output voltage of the same polarity. Buck or step-down converters are used to convert a higher DC input voltage to a lower DC output voltage of the same polarity. Forward converters are similar to buck-boost converters, but use a transformer to store energy and provide isolation between the input and output. The difference between flayback and forward transformers is in the way the energy transfer takes place. Switch mode transformers with a half-bridge and full-bridge output configuration are also available.
Switch mode transformers differ in terms of mounting configurations. Chassis-mounted devices use screw-down tabs. Disk-mounted devices use a simple rubber washer, a metal disc with a hole in the middle, and a sturdy clamping screw. H-frame mounting is used in applications with relatively high levels of shock or vibration. Modular jack designs ensure high common mode noise immunity while maintaining signal integrity. Switch mode transformers that mount on printed circuit boards (PCBs) use several mounting styles. Surface mount technology (SMT) adds components to a PCB by soldering component leads or terminals to the top surface of the board. Through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.
Certifications and Approval Agencies
There are many certifications and approval agencies for switch mode transformers. North American organizations include the American National Standards Institute (ANSI), the Canadian Standards Association (CSA), and Underwriters Laboratories (UL). European organizations include TÜV Rhineland Berlin-Brandenburg and the VDE Testing and Certification Institute. Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS will become effective on July 1, 2006. The Waste Electrical and Electronics Equipment Regulations (WEEE Regulations) is a European Parliament Directive that is designed to encourage the reuse, recycling and recovery of electrical and electronic equipment, and to improve the environmental impact and performance of this equipment. Other standards organizations for switch mode transformers include the International Electrotechnical Commission (IEC). Suppliers that meet U.S. military standards (MIL-STD) are also available.
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